This invention relates to a method for preparing catalysts useful in the manufacture of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly, it is directed to the preparation of catalysts suitable for producing maleic anhydride from saturated hydrocarbons in higher yields than heretofore possible.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
The prior art discloses a number of catalysts used in the conversion of organic feed stocks to maleic anhydride. As an example, U.S. Pat. No. 2,773,836 discloses phosphorus-vanadium-oxygen catalysts for the conversion of olefins to maleic anhydride. The catalysts had a weight ratio of V.sub.2 O.sub.5 to P.sub.2 O.sub.5 of 3:2 to 1:2, and were prepared by adding a vanadium compound to phosphoric acid, optionally adding a carrier to the solution, removing the excess liquid by evaporation, drying the remaining material at 200.degree. - 400.degree. F., grinding the resulting solids, and heating to 700.degree. -1100.degree. F. for several hours. U.S. Pat. No. 3,156,707 also discloses a similar method for preparing phosphorus-vanadium-oxygen catalysts for the conversion of olefins to maleic anhydride. The vanadium in these catalysts was reduced to an average valence in the range of 2.5 to 4.6 using an acid such as hydrochloric acid or oxalic acid during the preparatory steps.
Of particular interest is U.S. Pat. No. 3,293,268 which teaches a process of oxidizing saturated aliphatic hydrocarbons to maleic anhydride under controlled temperature conditions and in the presence of phosphorus-vanadium-oxygen catalysts. One method taught in that patent for preparing catalysts comprised reacting phosphoric acid with a vanadium compound in aqueous hydrochloric acid solution, recovering the remaining solids by evaporating the solution to dryness, and then heating the solids to 300.degree. to 500.degree. C. The resulting catalysts were ground to pass a 20 mesh screen and pelleted to form tablets. The tablets were then charged to a fixed catalyst bed in a test reactor at room temperature and the reactor heated for 16 hours. Thereafter, a 0.5 volumne percent butane in air mixture was passed through the catalyst in a fixed tube reactor at temperatures above 400.degree. C to form maleic anhydride.
Although yields in excess of 35 weight percent were reported in U.S. Pat. No. 3,293,268 when using low butane concentrations in air, these yields were achieved only at temperatures between 500.degree. C and 600.degree. C. At temperatures below about 500.degree. C, the yields of maleic anhydride were reported to be less than about 20 weight percent. On the other hand, the catalysts of the present invention can convert butane to maleic anhydride in significant yields at temperatures as low as 350.degree. C. Since it is well known to those skilled in the art that the more active catalysts can convert hydrocarbons to maleic anhydride at lower operating temperatures, the improved yields at lower operating temperatures that are achieved using the present catalysts show that the present catalysts are far superior to the prior art catalysts.